KR20020010584A - An aluminum back junction solar cell and a process for fabrication thereof - Google Patents

Abstract

The present invention relates to a method of manufacturing a solar cell. This manufacturing method includes providing a base layer 118 and a p-type conductive emitter layer 120 on the same side as the non-irradiated surface 128 of the base layer to provide a strongly doped p-type emitter layer. Forming a pn junction 124 between n-type base layer 118 and p-type emitter layer 120. The base layer of the present invention has n-type conductivity and is limited in scope by the irradiated surface 126 and the non-irradiated surface 128 on the opposite side of the irradiated surface.

Description

Aluminum Alloy Back Bonded Solar Cell and Manufacturing Method Thereof {AN ALUMINUM BACK JUNCTION SOLAR CELL AND A PROCESS FOR FABRICATION THEREOF}

Solar cells are widely used because they convert energy readily available from light sources such as the sun into electric power to operate electrically driven instruments such as calculators, computers, and household heaters. 1 illustrates a cross-sectional view of a layered stack constituting a conventional solar cell 10. Conventional silicon solar cells 10 typically have a pn junction 24 sandwiched between the p-type base layer 18 and the n-type layer 16, which is located near the irradiation surface (front side) 11. . The term "illuminated surface" as used herein refers to the surface of a conventional solar cell that is exposed to light energy when the solar cell is active or in operation. Thus, the term "non-illuminated surface" refers to a surface on the opposite side of the irradiation surface. The basic structure of the pn junction 24 includes an n-type emitter layer (n ⁺ ; 16) that is strongly doped (about 10 ²⁰ cm ^-3 ) or adjacent to the irradiated surface 11, which emitter layer is suitable. Is disposed over a heavily doped (about 10 ¹⁵ cm ^-3 ) p-type base layer (p; 18). Commercial embodiment of the conventional solar cells are typically selective anti-reflection, and a coating 14 and the p ⁺ layer 20, the p ⁺ layer of p-type base layer 18 and the p-type silicon contact 22 It is formed between.

The typical depth of the pn junction 24 from the n ⁺ emitter layer 16 is measured at about 0.5 μm. The shallow front surface pn junction 24 is advantageous for easily collecting minority carriers generated on both sides of the pn junction 24. Each photon of light that penetrates into the p-type base layer 18 and is absorbed by the base layer 18 transfers its energy to electrons in a confined state (covalently bonded state), freeing the electrons. These movable electrons and holes in the covalent bonds left behind (holes are also mobile) include a potential element of current flowing from the solar cell. To contribute to this current, the electrons and holes cannot be recombined, but rather separated by the electric field associated with the pn junction 24. When this happens, the electrons move to the n-type silicon contact 12 and the holes move to the p-type silicon contact 22.

In order to contribute to the current of the solar cell, photogenerated minority carriers (holes in the n ⁺ emitter layer and electrons in the p-type base layer) are sufficient to allow them to migrate by diffusion into the pn junction 24 where they are collected. It must exist for a long time. The average distance the minority carriers can travel without loss due to recombination with the majority carriers is referred to as the minority carrier spreading distance. Minority carrier diffusion distances generally depend on factors such as the density of defects (ie, recombination centers) in the silicon crystal and the density of dopant atoms in the silicon. As the density of defects or dopant atoms increases, minority carrier diffusion distances decrease. Thus, the diffusion distance of the holes of the strongly doped n ⁺ emitter layer 16 is much smaller than the diffusion distance of the electrons of the properly doped p-type base layer 18.

One of ordinary skill in the art recognizes that the n ⁺ emitter layer 16 is almost a "dead layer," in which the minority charge carriers caused in the emitter layer 16 lose pn junctions without loss due to recombination. It can hardly spread to 24). For various reasons, it is desirable to have n ⁺ emitter layer 16 as shallow or possibly as close to the surface of emitter layer 16. For example, the shallow emitter layer allows relatively few photons to be absorbed into the n ⁺ emitter layer 16. In addition, the final photogenic minority carriers caused by the n ⁺ emitter layer 16 are themselves sufficiently adjacent to the pn junction 24 to have a theoretical opportunity to be collected (diffusion distance ≧ junction depth).

Unfortunately, in conventional solar cell structures, the depth of the n ⁺ emitter layer is limited and cannot be as shallow as desired. Metal from the emitter contacts 12, in particular the contacts formed by screen-printing and firing, can penetrate the pn junction 24, breaking or damaging the junction. The presence of the metal in the pn junction 24 "shorts" or "shunts" the junction. Therefore, the shallower and less doped n ⁺ emitter layer 16 is advantageous for increasing the current generated by the cell, but in practice the n ⁺ emitter layer 16 is more pn junction 24 than if it was relatively deeply doped. It is not desirable to prevent shunting. As a result, in a conventional solar cell, if the position of n ⁺ emitter layer 16 is deep, the amount of current generated by the cell is reduced.

Therefore, there is a need for a structure of a silicon solar cell and a method of manufacturing the same, which have a long minority carrier diffusion distance, prevent the classification of p-n junctions, and do not reduce the amount of generated current.

The present invention relates to an improved solar cell and a method of manufacturing the same. More specifically, the present invention relates to a solar cell in which a p-n junction is located near the non-irradiated surface of the solar cell and a method of manufacturing the same.

1 is a cross-sectional view of a conventional solar cell,

2 is a cross-sectional view of a solar cell according to an embodiment of the present invention,

3A to 3D are views illustrating various steps of manufacturing the solar cell of FIG. 2 according to an embodiment of the present invention.

4 is a graph of the I-V curve measured for the solar cell of the dendritic web bonded to the aluminum alloy back,

5 is a graph showing the internal quantum efficiency of the solar cell of the dendritic web bonded aluminum alloy back;

6 is a cross-sectional view of a solar cell according to another exemplary embodiment of the present invention, in which a metal partially covers a rear surface of the solar cell,

FIG. 7 is a cross-sectional view of a solar cell according to another embodiment of the present invention in which the contact with the base layer is in an interdigitated fashion at the back of the cell.

In one aspect, the present invention provides a solar cell. The solar cell includes a base layer with n-type conductive dopant atoms, the base layer being limited in scope by the irradiated and non-irradiated surfaces. The irradiated surface is the surface where the light energy collides when the solar cell is exposed to the light energy, and the non-irradiated surface is on the opposite side of the irradiated surface. The solar cell further includes a backside emitter layer made of an aluminum alloy and functioning as a p-type conductive layer. The solar cell further has a p-n junction layer disposed between the non-irradiated side of the base layer and the emitter layer on the back side.

In another aspect, the present invention provides a method of manufacturing a solar cell. The method comprises the steps of (1) providing a base layer, and (2) forming a p-type conductive emitter layer on the same side as the non-irradiated surface of the base layer to provide a strongly doped p-type emitter layer, Forming a pn junction between the base layer and the p-type emitter layer. The base layer of the present invention has n-type conductivity and is limited in scope by the irradiated and non-irradiated surfaces. The irradiated surface is the surface where the light energy collides when the solar cell is exposed to the light energy, and the non-irradiated surface is on the opposite side of the irradiated surface.

These and other features of the invention are described in detail in the following examples with reference to the accompanying drawings.

The present invention will now be described in detail with reference to a preferred embodiment of the invention illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to assist in a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details. On the other hand, well known process steps and / or structures are not elucidated since they may obscure the present invention.

The diffusion distance of minority carrier electrons (ie electrons in p-type silicon) is generally different from the diffusion distance of minority carrier holes (ie holes in n-type silicon) for a given doping density. This difference may depend on the nature of the crystal defects that can act as recombination centers. In silicon of dendritic webs, for example, the main defect is the precipitation of silicon oxide nucleating at the dislocation core. The interface between the silicon oxide precipitate and the surrounding silicon carries a positive charge because of the mismatch in the atomic structure of the two materials. Negatively charged minority carrier electrons lead to this positively charged interface. These defects can trap and retain electrons until one of the many majority carrier holes roams by the defect, so that in the case of recombination the electrons can fall into the holes.

Therefore, according to the present invention, it can be seen that, for the substrate layer of the solar cell, n-type doping is advantageous for obtaining a minority carrier diffusion distance larger than the diffusion distance obtainable by p-type doping. Solar cell substrates with n-type conductivity require an n-type base layer. Therefore, the p-n junction was formed by employing an emitter layer and an n-type base layer on the irradiation side including boron, which is a practical choice for p-type dopants.

Unfortunately, boron diffusion has been found to frequently leave stained silicon surfaces due to the formation of boron-silicon compounds. These stains seriously damage the appearance of the solar cell surface. Boron stains may be removed by using additional unwanted processing steps such as thermally oxidizing the stained surface followed by chemical removal of the heating oxide. Thus, according to the findings of the present invention, for silicon materials where n-type doping provides a minority carrier diffusion distance than p-type doping, the front p-n junction is undesirable due to the presence of boron stains in the p-type emitter layer.

Referring again to FIG. 1, commercial solar cell 10 has an n ⁺ pp ⁺ structure. The n-type emitter layer 16 is typically formed using diffusion that is (no stain is a problem), the silicon substrate is doped p-type, and the back silicon surface 20 is in contact with the contact metal layer 22. Is p ^{+ which} is typically doped to enhance the ohmic contact of. Ohm contact has low resistance. The voltage generated across the ohmic contact has a linear relationship with the current flowing through the contact. In contrast, the voltage generated across the rectifying contact has a nonlinear relationship with the current flowing through the contact. Aluminum-silicon alloys can be used to provide p ⁺ doping and back metal contacts. More importantly, commercial solar cells 10 have a pn junction 24 located in front of the cell. Therefore, as described above, n ⁺ emitter layer 16 is forced deeper than is required to prevent classifying the forward junction. When a silicon material having good minority carrier diffusion distance with n-type doping is used with the front pn junction, the p-type emitter layer is substantially boron doped. As noted above, boron doping leaves a stained surface that damages the appearance of the cell and can only be removed using additional unwanted treatment.

Therefore, in the present invention, the pn junction is located at the rear end of the solar cell. 2 shows a layer stack 110 of a solar cell according to one embodiment of the invention, in which the pn junction is located at the rear end (near the non-irradiation surface of the base layer), and thus the front metal contact 112. It is impossible to classify this pn junction. Since aluminum can be selected as a dopant in the silicon p ⁺ layer, it is also impossible for aluminum to classify the bond.

The layer stack 110 of the solar cell includes an n-type conductive base layer 118, which is limited in scope by the irradiated surface 126 (front or top) and the non-irradiated surface 128 (back or bottom). do. A portion of the base layer 118 near the irradiation surface 126 is immobilized by the shallow n + layer 116, which is typically formed by diffusing phosphorus into the silicon layer. Therefore, this embodiment realizes the advantages of the conventional n ⁺ layer, and there is no problem that boron stain remains. An optional antireflective (AR) coating 114 is disposed on top of n ⁺ layer 116. If an antireflective coating is present as shown in FIG. 2, the silver contact 112 is fired through the AR coating 114 to reach the surface of the n ⁺ silicon layer 116.

The pn junction 124 is formed at the non-irradiation surface 128 or at the interface between the base layer 118 and the aluminum doped silicon layer 120, which layer of p ^{+ is} already in the layer stack of the solar cell of the present invention. Acts as a ground layer. The aluminum-silicon eutectic metal layer 122, which occurs when aluminum is alloyed with silicon, is disposed below the p ⁺ layer 120 and acts as a self-aligned contact to the p ⁺ emitter layer 120. It is also important that there is no boron doping in the solar cell structure of FIG.

Placing the pn junction on the back of the cell 110 is a unique feature of the solar cell 110 shown in FIG. It should be noted that no current silicon solar cell structure employs a back junction of this aluminum alloy. In addition, the depth of the n ⁺ layer 116 does not have any problem with fractionation since the pn junction 124 is at the back of the cell 110.

According to one embodiment, the n-type base layer 118 is generally a silicon crystal of a dendritic web having a resistivity of about 5 Pa.cm to about 100 Pa.cm, preferably about 20 Pa.cm. The thickness of the silicon based crystals of the dendritic web is generally from about 30 μm to about 200 μm, preferably about 100 μm. In order to allow the cell 110 to operate at a practical energy conversion efficiency level, in one example of this embodiment, the hole diffusion distance in the n-type base layer 118 is slightly less than the thickness of the base layer 118. However, in a preferred example, the diffusion distance of the holes is substantially the same as the thickness of the base layer 118, and more preferably exceeds the thickness of the base layer 118.

Placing the p-n junction near the back of the cell instead of near the front of the cell is not intuitive to one skilled in the art. It is generally desirable to place the p-n junction near the front side of the cell because the number of photons absorbed per unit depth decreases as it progresses from the irradiated side to the back side. For example, about 52% of incident photons are absorbed within the first 5 μm of the base layer from the irradiated surface, while about 10% are absorbed within the second 5 μm portion of the depth and about 6% are the third 5 μm portions. Are absorbed within. In fact, about 75% of the incident photons are absorbed within the top 30 μm of the base layer.

Because of the absorption of photons, a pair of electron-holes is generated, and the minority carrier members of the pair diffuse and collect at the junction, so it is reasonable to place the p-n junction near the irradiation surface. Experience has shown that these minority carriers beyond one diffusion distance from the junction are not collected and thus do not contribute to the photocurrent of the solar cell.

3A-3D illustrate important intermediate solar cell structures formed at different stages of the manufacturing process according to one embodiment of the invention. Conventional techniques in which n-type conductive single crystal silicon substrates are well known, such as Czochralski, Float-zone, Bridgman, EFG (edge defined film-fed growth), string ribbon When manufactured using techniques such as string ribbon growth, silicon casting crystallized by bidirectional solidification, dentritic web growth, an exemplary embodiment of the process of the present invention may be initiated. However, in a preferred embodiment, dendritic web growth techniques can be performed to form n-type single crystal substrates.

Referring to FIG. 3A, the n-type single crystal functions as the base layer 218, under which the pn junction of the solar cell is formed sequentially. In the next step, a heavily doped n-type layer (216 (hereinafter referred to as n ⁺ layer)) will be formed on top of base layer 218 using a diffusion process well known to those skilled in the art of semiconductor and solar cell manufacturing. Can be. According to one embodiment of the invention, a layer containing phosphorous is coated on the top surface of the base layer 218, and phosphorus atoms are pushed into the base layer 218 from any heat source. The layer of phosphorus is preferably obtained from a screen printed paste or liquid dopant, and the heat source is a rapid heat treatment (RTP) unit or a belt furnace maintained at a temperature of about 750 ° C. to about 1050 ° C., preferably about 950 ° C. )to be. At these temperatures, the diffusion time can generally be from about 30 seconds to about 30 minutes, preferably about 5 minutes. As a result, a doped n + passivating layer having a thickness of generally from about 0.1 μm to about 1 μm, preferably about 0.3 μm, is formed, and the area resistance is generally from about 10 mW / square to about 200 mW / square, preferably Is 40 mW / square.

Optionally, in another embodiment, phosphorus ions are implanted into the top surface of the base layer 218 and a portion near the top surface of the base layer is converted to n ⁺ layer 216.

According to another embodiment, the base layer 218 is immobilized by a transparent dielectric layer that may have a distinct positive or negative charge. In one example of this embodiment, the base layer is immobilized by a layer of thermally grown silicon dioxide having a thickness of about 50 GPa to about 500 GPa, preferably about 150 GPa. In this embodiment, passivation of the base layer 218 may be performed in a rapid heat treatment unit at a temperature of about 800 ° C to about 1050 ° C, preferably about 1000 ° C. This heat treatment is generally carried out for about 30 seconds to about 30 minutes, preferably about 50 seconds. Regardless of how the n ⁺ layer is immobilized, or whether the layer is immobilized, those skilled in the art will appreciate that due to the formation of the n ⁺ layer 216, a high-low junction n ⁺ n It can be seen that is formed on top of the base layer 218.

Next, an antireflective coating 214 may be coated on top of n ⁺ layer 216 as shown in FIG. 3B. For example, to form the antireflective coating 214, plasma enhanced chemical vapor deposition (PECVD) of silicon nitride or atmospheric pressure chemical vapor deposition (APCVD) of titanium dioxide is performed.

Next, conventional techniques are used to deposit the aluminum layer 222 on the bottom surface or non-irradiation surface of the base layer 218. According to one embodiment of the invention, aluminum is deposited by screen printing pure aluminum paste on the bottom surface of the base layer 218. In one example of this embodiment, aluminum layer 222 is laminated by screen printing to a thickness of about 5 μm to about 30 μm, preferably about 15 μm, over almost the entire backside of the cell. The partially fabricated solar cell is then sufficiently long at a suitable temperature above the aluminum-silicon eutectic temperature of about 577 ° C., such as generally from about 700 ° C. to about 1000 ° C., preferably about 800 ° C., in a radiant heated belt furnace. For a duration of time, for example, generally from about 0.1 minutes to about 10 minutes, preferably about 2 minutes, it is possible to easily alloy at least some aluminum alloy from layer 222 with base layer 218. During the alloying process, the heat treatment inside the belt furnace is high enough that aluminum dissolves the silicon efficiently. At this high temperature, a portion of the base layer, measured about one third of the thickness of the aluminum layer, dissolves efficiently, forming the p ⁺ layer 220 shown in FIG. 3C. For example, at about 800 ° C., an aluminum layer 222 having a thickness of 15 μm dissolves a 5 μm portion of the silicon base layer. As a result, the p ⁺ layer 220 is silicon doped with aluminum, and the contact layer 222 is an alloy having a eutectic composition of about 88 wt% aluminum and 12 wt% silicon.

The p ⁺ emitter layer is prepared as described below. As the silicon cools, the silicon continues to solidify or grow in the underlying silicon, employing an appropriate amount of aluminum dopant with about 10 ¹⁸ to about 10 ¹⁹ aluminum atoms per cm 3. The density of aluminum atoms is significantly less than that of silicon atoms, which is about 5x10 ²² per cm 3, about 10,000 times. As shown in FIG. 3C, after cooling the layer, a portion of the base layer 218 near the non-irradiation surface is heavily doped p-type layer 220 (hereinafter referred to as p + layer) by liquid phase epitaxy. Is referred to as). Some of the dissolved silicon becomes part of the aluminum-silicon eutectic layer 222. Silicon not dissolved by aluminum acts as a template to reconstruct silicon crystals when it is epitaxial regrowth.

The aluminum-silicon eutectic metal layer 222 is a good conductor (low electrical resistance) and functions as a self aligned metal contact for the solar cell. The thickness of the aluminum-silicon eutectic metal layer 222 is generally from about 1 μm to about 30 μm, preferably about 15 μm.

3d shows the resulting structure after laminating metal contacts and firing through an optional antireflective coating. For example, a layer of silver is screen printed on n ⁺ layer 216 so as to surround about 7% of the front side in a lattice pattern. Silver may be laminated by screen printing to a thickness of between about 5 μm and about 20 μm, preferably about 10 μm. The resulting partially fabricated solar cell structure then travels through a belt furnace that is generally maintained at a temperature of about 700 ° C to about 900 ° C, preferably about 760 ° C. The time that the cell runs through the furnace is short to prevent the diffusion of impurities in the silver paste into the silicon. In one embodiment of the invention, the time the battery runs through the belt furnace is from about 0.1 minutes to about 10 minutes. However, in a preferred embodiment, the travel time is about 40 seconds. The thickness of the ohmic contacts 212 of silver leading to the n ⁺ layer 216 is generally from about 1 μm to about 20 μm, preferably about 10 μm. When an optional antireflective coating is laminated, the layer of silver is fired through the coating. Both operations (with or without anti-reflective coating) can be performed in a radiant heated belt furnace. The manufacturing step of the present invention realizes high yield and low manufacturing cost.

FIG. 4 shows an IV curve showing that the efficiency of converting light energy into electrical energy is about 12.5% in a dendritic web silicon solar cell having an aluminum alloy back-bonded in its entirety with metal according to the present invention. The solar cell employed to obtain the data shown in FIG. 4 has a nominal thickness of about 100 μm and an area of about 25 cm 2. Other parameters include a short circuit current density of about 28.2 mA / cm 2, an open circuit voltage of about 0.595 V, and a fill degree of about 0.747. The knee portion of the curve represents the maximum power point, where V _mp is about 0.480 V and I _mp is about 0.653 A.

FIG. 5 shows measurements of the spectral reflectance (510 (solid triangle), external quantum efficiency 512 (empty square), and internal quantum efficiency 514 (solid square) for the solar cell of FIG. 4. The reflectance measured indicates that about 47% of the long wavelength light (greater than 950 nm) is reflected from the back side of the aluminum-silicon eutectic metal layer. This reflected light then passes through the silicon a second time, increasing the overall efficiency. The calculated value of internal quantum efficiency using a primary one-dimensional finite element model PC1D indicates that the hole diffusion distance at the base of this cell is about 200 μm. This is twice the cell thickness required for good performance. The internal quantum efficiency curve has a positive slope over the range of about 500 nm to about 850 nm, which means that the p-n junction is at the back of the cell, which is also important. In conventional solar cells, the internal quantum efficiency curve shows a negative slope, which means that the p-n junction is at the front of the cell.

FIG. 6 shows a layer stack 300 of solar cells according to another embodiment of the present invention, except that only a portion of the solar cell 300 is converted to a p-type layer and surrounded by metal. It is substantially similar to cell 110. As such, solar cell 300 is " partial " as opposed to referring to solar cell 110 of FIG. 2, the entire backside of which is surrounded by metal, as a solar cell having a " full " metal cover. It is referred to as a solar cell with a metal cover. The structure with the partial metal sheath of FIG. 6 prevents warpage of thin cells, resulting from different thermal shrinkage between silicon and aluminum-silicon eutectic metal layers when cooled from an eutectic temperature of about 577 ° C. It is also important to remember that such warpage often occurs when the entire metal sheath is used. Segmented metal also reduces the cost of aluminum. Surface 330 of the exposed n-type base layer between metal islands on the backside may be passivated to obtain significant energy conversion efficiency. For example, the passivation step may be performed after the aluminum alloying step by growth of a heating oxide having a thickness of about 115 kPa in a rapid heat treatment unit at about 1000 ° C. for about 50 seconds. The antireflective coating may not be coated in this preliminary step. An efficiency of up to about 6.8% was obtained for a 25 cm 2 dendritic web silicon cell with screen printed metal and no antireflective coating. The expected efficiency with an antireflective coating is about 9.9%.

FIG. 7 shows a layer stack 400 of solar cells according to another embodiment of the present invention, wherein the solar cell 400 is formed of a cell in such a manner that the n-type contacts 412 for the n-type base layer are interdigitated. It is substantially similar to the solar cell 300 of FIG. 6 except that it is moved from the front to the back of the cell.

This structure, shown in FIG. 7, has the further advantage of reducing the shadows on the front face by the contacts to zero. Interconnecting the cells in the module is expected to be simpler as it has both contacts on the back side of the cell, and surface mount technology can be used. The n ⁺ region 424 adjacent to the silver contact 412 may be formed in a separate step. Optionally, the source of this n-type dopant may be the contact material itself when a self-doped contact metal such as a silver-antimony alloy is used instead of pure silver. This type of prototype dendritic web silicon solar cell with a screen printed metal, an antireflective coating of titanium dioxide and an area of about 2.5 cm 2, had an efficiency of about 10.4%.

Although the present invention has been described with reference to silicon crystal substrates and aluminum, it is also important that other substrates and conductive materials can be similarly employed. In addition, new and novel structures and processes may be disclosed for producing solar cells with aluminum alloy back bonded. In accordance with the present invention, those skilled in the art can understand that there are many variations and equivalents employing the invention disclosed herein. As a result, the present invention is not limited to the above-described exemplary embodiments, but only by the following claims.

Claims (39)

A base layer containing a dopant atom of n-type conductivity and limited in scope by an irradiated surface and a non-irradiated surface, wherein the irradiated surface is a surface to which light energy collides when the solar cell is exposed to light energy, and the non-irradiated surface is irradiated Such a base layer on the opposite side of the face, A back emitter layer comprising contacts of an aluminum alloy and acting as a layer of p-type conductivity, A p-n junction layer disposed between the irradiation surface of the base layer and the back emitter layer Solar cell comprising a. The solar cell of claim 1, further comprising a heavily doped n-type layer disposed adjacent the irradiation surface of the base layer. The solar cell of claim 2, wherein the heavily doped n-type layer comprises phosphorus atoms. 3. The solar cell of claim 2 further comprising an antireflective coating layer disposed adjacent the strongly doped n-type layer. 5. The solar cell of claim 4, further comprising a metal contact that is fired through the antireflective coating down to the heavily doped n-type layer. The solar cell of claim 5 wherein the metal contact comprises silver and functions as an ohmic contact for the base layer. The solar cell of claim 5, wherein the metal contact has a thickness of about 1 μm to about 20 μm. The solar cell of claim 1, wherein a diffusion distance of holes through the base layer is slightly smaller than a thickness of the base layer. The solar cell of claim 8, wherein a diffusion distance of holes through the base layer is substantially equal to a thickness of the base layer. The solar cell of claim 9, wherein a diffusion distance of holes through the base layer exceeds a thickness of the base layer. The method of claim 10, wherein the n-type base layer is silicon casting crystallized by bidirectional solidification, edge defined film-fed growth, string ribbon growth, Zzochralski method (Czochralski) A solar cell, characterized in that it is manufactured using a technique selected from the group consisting of Float-zone, bridgeman method and dentritic web growth. 12. The solar cell of claim 11 wherein the n-type base layer is a dendritic web silicon layer. 13. The solar cell of claim 12 wherein the resistivity of the n-type base layer is between about 5 Pa.cm and about 100 Pa.cm. 13. The solar cell of claim 12 wherein the resistivity of the n-type base layer is about 20 kPa.cm. The solar cell of claim 12, wherein the thickness of the n-type base layer is between about 30 μm and about 200 μm. The solar cell of claim 15, wherein the n-type base layer has a thickness of about 100 μm. The solar cell of claim 1 wherein the aluminum alloy layer acts as a self-aligned contact with respect to the non-irradiated surface of the emitter layer. 18. The solar cell of claim 17 wherein the aluminum alloy layer is an aluminum-silicon eutectic metal layer. The solar cell of claim 17, wherein the aluminum-silicon eutectic metal layer has a thickness between about 1 μm and 30 μm. 18. The solar cell of claim 17 wherein the aluminum alloy layer substantially surrounds the non-irradiated surface of the base layer. 18. The solar cell of claim 17 wherein the aluminum alloy layer partially surrounds the non-irradiated surface of the base layer. 22. The method of claim 21, wherein the aluminum alloy layer partially surrounds the non-irradiated surface of the base layer in a divided manner, wherein the interdigitated n-type metal contact is disposed in the space between the divided aluminum alloy layers. Solar cells. 23. The solar cell of claim 22 further comprising an n-type layer strongly doped adjacent to a metal contact to facilitate connection to the base layer. A base layer having an n-type conductivity and defined by a range of irradiation and non-irradiation surfaces, wherein the irradiation surface is a surface to which light energy collides when the solar cell is exposed to light energy, and the non-irradiation surface is opposite to the irradiation surface. Preparing such a base layer, Forming a p-type conductive emitter layer on the same side as the non-irradiated surface of the base layer to provide a heavily doped p-type emitter layer, thereby forming a p-n junction between the n-type base layer and the p-type emitter layer Solar cell manufacturing method comprising a. The method of claim 24, wherein the preparing of the base layer comprises: silicon casting crystallized by Czochralski, Float-zone, bridgeman method, bidirectional solidification, EFG method [ edge defined film-fed growth], including producing a n-type conductive single crystal silicon substrate using a technique selected from the group consisting of string ribbon growth and dentritic web growth. A solar cell manufacturing method characterized by the above-mentioned. The method of claim 25, wherein preparing the base layer comprises fabricating an n-type conductive single crystal silicon substrate using dendritic web growth techniques. The method of claim 24, wherein providing the emitter layer is Providing an aluminum layer on the non-irradiated surface of the base layer; Alloying the aluminum from the back with the portion of the base layer to convert at least a portion of the base layer to the emitter layer Solar cell manufacturing method comprising a. 28. The method of claim 27, wherein the base layer comprises silicon and the alloying step is self-aligned with respect to the emitter layer comprising aluminum atoms as a p-type dopant and the backside of the emitter layer comprising aluminum-silicon alloys. A solar cell manufacturing method comprising forming a contact layer. 29. The method of claim 28, wherein providing the aluminum layer comprises screen printing aluminum on the non-irradiated side of the base layer. 29. The method of claim 28, wherein said alloying step is performed at a temperature between about 700 ° C and about 1000 ° C. The method of claim 28, wherein said alloying step is performed by maintaining the temperature for about 1 minute to about 30 minutes. The method of claim 28, wherein the irradiated surface of the base layer is immobilized by a phosphorus doped layer having a thickness of about 0.1 μm to about 1 μm. 33. The method of claim 32, further comprising forming an antireflective coating on the phosphorus doped layer, preferably comprising titanium dioxide or silicon nitride. The method of claim 33, wherein the n-type metal contact is formed over the base layer to provide ohmic contact to the base layer. 35. The method of claim 34, wherein an n-type metal contact is formed under the base layer to provide ohmic contact to the base layer. 28. The method of claim 27, wherein providing the aluminum layer comprises providing an aluminum layer such that the aluminum alloy layer formed in the alloying step substantially surrounds the backside of the emitter layer. . 28. The method of claim 27, wherein providing the aluminum layer comprises forming an aluminum layer partially surrounding the non-irradiated surface of the base layer such that the alloying step forms an emitter layer that encloses the non-irradiated surface of the base layer in a divided manner. The solar cell manufacturing method characterized by including providing. 38. The method of claim 37, further comprising forming an interdigitated n-type metal contact disposed in the space between the cross-sectional aluminum alloy layers. 25. The method of claim 24, further comprising passivating substantially all of the exposed and non-irradiated surfaces of the base layer with a dielectric layer.